This invention relates to methods and means for eliminating corona in high voltage generators or other electrical machines having exterior coil windings disposed in air or other gaseous medium. The dangers of corona, in high voltage machines are well known. High voltage stator coils generally require the use of electrical stress grading systems along the exterior end portions of the coil for corona suppression. Without stress grading, the electric field along the surface of the coil can become sufficiently large so that the air layer adjacent to the coil can break down and flashover from the high voltage leads to the grounded machine can result.
Several methods of preventing corona discharge and short-circuiting have been used. One method, taught by Philofsky et al., in U.S. Pat. No. 3,823,334, consists of embedding epoxy resin strips containing barium titanate filler, within the coil insulation. The high dielectric strips are arranged so that predetermined intermediate voltage belts control the electric field within the insulation, and the surface stress does not reach too large a value. This method, however, requires precise positioning of the strips and a very high measure of quality control.
A simpler stress grading system involved the use of high resistance paint films on the insulation of the coil end portion, as taught by Hill et al., in U.S. Pat. No. 2,318,074. There, the entire insulated stator coil was painted with a 10 to 20 mil thick coating of a partially conducting material. The coating consisted of a varnish binder, and a major portion of a finely divided powder selected from carbonized cellulose, i.e., wood-char, and partially reduced titanium dioxide. After the varnish was applied, and while it was still wet, the coil was wrapped with a porous cotton, asbestos or braided glass reinforcement tape, so that the paint penetrated the tape. The impregnated tape was coated with a final coat of the same or a similar varnish. Thereafter, the slot portion of the coil could be overcoated with a relatively conducting paint. This method, however, provided a linear type varnish coating which made it useful primarily for low voltage machines, i.e., under about a 20 KV voltage class.
Later developed film coating systems, as taught by Berg et al., in U.S. Pat. No. 3,210,461, involved coating insulated, exterior stator coil portions next to the grounded stator laminations with a 10 mil thick coating of a semiconducting material. This material consisted of 1 part varnish binder and about 6 parts of finely divided non-linear silicon carbide powder, containing up to 4 wt.% of finely divided carbon. The resistivity of these silicon carbide coatings was non-linear, i.e., the resistivity varied with the voltage, whereas the resistivity of wood-char coatings was linear and remained constant irrespective of voltage. The exterior coil portions were simply coated with a single layer of the paint and dried. This method provided a somewhat brittle coating and was useful primarily in medium voltage machines operating at between about a 20 to 25 KV voltage class.
Broeker et al., in U.S. Pat. No. 3,354,331, taught non-linear silicon carbide in a suitable binder applied as a tape near the loop end portion of the stator coil. In a further development, S. Hirabayaski et al. in Proceedings of the 12th Electrical/Electronics Insulation Conference, "New Corona Suppression Method For High Voltage Generator Insulation", 1975, pp. 139-142, taught a more sophisticated grading system involving a double layer of a similar type non-linear semiconducting silicon carbide paint, each layer being insulated from the other by a thick, epoxy resin impregnated, mica paper separator.
In the Hirabayaski et al. teaching, the inner non-linear silicon carbide paint contacted the base conducting varnish at a base junction point. The outer layer of non-linear silicon carbide paint contacted the base conducting paint, through a conducting paint layer over the mica paper separator, at a second junction point displaced a critical distance down the coil tangent from the base junction point. This provided a grading system having somewhat improved failure voltages of about 80 KV, but the thickness of the stress grading system was substantially increased to about 175 mils, and quality control of the critically displaced junction points between the conducting varnish and the non-linear silicon carbide paint layers was difficult.
What is needed is a voltage grading system having a cross section no greater than about 150 mils, to provide more clearance between the assembled coil tangents, in combination with higher failure voltages on the order of 95 KV. Such a system could be used on high voltage machines operating at about a 35 KV voltage class.